Data Management: Queries
ArcPlot commands for selecting and summarizing data
Meet certain conditions: reselect, nselect, aselect, unselect
Determine characteristics of a specified feature: identify
Perform limited statistical operations on the database: statistics
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Overview
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The manipulation of coverages lies at the heart of ArcInfo-based GIS
The methods or commands are numerous and can be classified in many ways
There is always overlap, but basic groupings are
– Coverage Query: techniques for querying existing coverages (see query.ppt)
– Coverage Maintenance: creating, maintaining and modifying coverages (see covergages.ppt)
– Coverage Conversion: converting from one GIS format or data model to another (see
convert.ppt)
– Spatial Analysis: manipulation of coverages to answer some research question or achieve
some operational purpose (see analysis.ppt)
• Spatial analysis techniques may be used for coverage query anmd maintenace also
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We will discuss Query and Maintenance in the context of Workstation ArcInfo.
– query.ppt
– coverage.ppt
ArcToolbox includes these in its Data Management component
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We will cover the Conversion and Spatial Analysis commands in the context of
ArcToolbox,
– Conversion.ppt
– Analysis.ppt
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SPATIAL QUERIES
• ARCPLOT commands which allow the user to display and
query spatial data.
– Determine available data:
• items
• list
– Meet certain conditions
• reselect
• nselect
• aselect
• unselect
– Determine characteristics of a specified feature
• identify
– Perform limited statistical operations on the database
• statistics
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Spatial Queries: Selecting Records
When you select a table, or draw a coverage in ArcPlot, all records are
contained in the active set and any commands executed will affect all the
records in the table. To modify specific records, you need to place records in a
subset:
– RESELECT - selects a subset of records from the currently selected data set using a
variety of methods -- see usage for reselect in ARCPLOT. Most popular: logical
expression, graphical (box, circle, etc), and overlap.
– ASELECT – when issued with logical expression, selects additional records to add
to selected set
-when issued alone, selects all records
– UNSELECT - removes features that match a logical expression
– NSELECT - switches the set of selected and unselected records.
This is critical.
Normally issue–
before every
reselect!
CLEARSELECT - clear the selected set -- not automatic, must clear the selected
set prior to reselecting another set unless only want a record from the currently
selected set.
– WRITESELECT - write the selected set into a selection file which can be read later
or used to create a new coverage in ARC.
– READSELECT - will read a set specified with the writeselect command
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RESELECT
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Arcplot: usage reselect
RESELECT <cover> <feature_class> {logical_expression}
RESELECT <cover> <feature_class> ONE <xy | *>
RESELECT <cover> <feature_class> MANY <xy...xy | *>
RESELECT <cover> <feature_class> BOX <xmin ymin xmax ymax |
*> {PASSTHRU | WITHIN}
RESELECT <cover> <feature_class> POLYGON <xy...xy | *>
{PASSTHRU | WITHIN}
RESELECT <cover> <feature_class> CIRCLE <xy radius | *
{radius}> {PASSTHRU | WITHIN}
RESELECT <cover> <feature_class> MAPEXTENT
RESELECT <cover> <feature_class> PARTIAL
RESELECT <cover> <feature_class> KEYFILE <info_key_file>
<item> {KEYITEM <key_file_item>}
RESELECT <cover> <feature_class> KEYFILE <cover>
<feature_class> <item> {KEYITEM <key_file_item>}
RESELECT <cover> <feature_class> OVERLAP <overlap_cover>
<overlap_feature> {selection_distance} {PASSTHRU | WITHIN}
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RESELECT, continued
• RESELECT <cover > <feature_class> RANDOM <number>
{PERCENT} {seed}
• RESELECT <info_file> INFO {logical_expression}
• RESELECT <info_file> INFO KEYFILE <info_key_file> <item>
{KEYITEM <key_file_item>}
• RESELECT <info_file> INFO KEYFILE <cover> <feature_class>
<item> {KEYITEM <key_file_item>}
• RESELECT <info_file > INFO RANDOM <number> {PERCENT}
{seed}
• RESELECT <image_catalog> IMAGE {logical_expression}
• RESELECT <image_catalog> IMAGE ONE <xy | *>
• RESELECT <image_catalog> IMAGE MANY <xy...xy | *>
• RESELECT <image_catalog> IMAGE BOX <xmin ymin xmax ymax
| *> {PASSTHRU | WITHIN}
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Logical Expressions
AND
OR
EQ
LE
NE
CN
Item name
Item name
GT
NC
Character string*
operand
GE
IN
numeric value
LT
LK
*Must be in ‘single quotes’
(And must use key next to Enter key)
operator
Equals, not equal to, greater than, greater than or equal to, less than,
less than or equal to, contains, not contains, in a range of values, like
Must have a space on each side of the operator, even when using
equal (=) symbol e.g. State = ‘TX’
Examples
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Select from coverage USA polygon features for the State of Texas:
– RESELECT <cover> <feature_class> {logical_expression}
– reselect usa poly state = ‘TX’ (reminder:Character strings are always in single quotes)
– RESELECT <cover> <feature_class> POLYGON <xy...xy | *> {PASSTHRU |
WITHIN}
– reselect usa poly polygon * within
(drag cursor box around Texas)
– RESELECT <cover> <feature_class> OVERLAP <overlap_cover>
<overlap_feature> {selection_distance} {PASSTHRU | WITHIN}
– reselect usa poly OVERLAP texas poly (already have a cover named texas)
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Find all buildings within 100’ of a stream
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REMEMBER: Once you have selected a set, all future commands act only on
the selected set. To clear the selected set use the command CLEARSELECT.
– RESELECT <cover> <feature_class> OVERLAP <overlap_cover>
<overlap_feature> {selection_distance} {PASSTHRU | WITHIN}
– reselect bldg poly overlap stream line 100 (assumes mapunits are in feet)
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Saving Selected Sets: writeselect v. new coverage
In ArcPlot (be sure to see big difference between Arc and ArcPlot use of reselect)
• Select records from coverage:
Arcplot: reselect <cover> <feature class> {logical selection}
Arcplot: reselect usa poly state eq ‘TX’
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Create a selection file for this selected set
Arcplot: writeselect <selection_file> {<cover> <feature class>}
Arcplot: writeselect texas.sel
Arcplot:
quit
This will create a selection file containing all current selection criteria (but no data
records). Use readselect to read this selection file another time to re-establish
these criteria.
In Arc
Use reselect command in Arc to create new coverage
Arc: reselect <in_cover> <out_cover> {in_feature class} {selection file}
{out_feature class}
Arc: reselect usa texas poly texas.sel poly
This will create a new coverage containing actual data records. Similar to
“Convert to Shapefile” in ArcView. However, rarely need to create a new
coverage -- use an AML to reselect your records in ArcPlot, or readselect a 9
writeselected set.
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READSELCT
• reads a previously created selection file and re-establishes the selected
record set based on the selection file contents.
• READSELECT <selection_file>
• For example, the following reads an existing selection file, which is
assumed to select only Oklahoma counties, so that the map extent will
be limited to Oklahoma rather than all US and only Oklahoma will
appear on the map. (uscnty is a coverage)
readselect okcnty.sel
mape poly uscnty
polys uscnty
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Retrieving Information About your Data
The IDENTIFY command allows you to point at a coverage feature and
display its attributes.
Usage: identify <cover> <feature_class> <xy | *>
Arcplot: identify site polys *
Enter point
Record AREA PERIMETER PARCEL# PARCEL-ID
17
2432.473
901.877
23
23
PIN
21392904
If you only want IDENTIFY to list one or more items, specify item
names:
Arcplot: identify site polys * area pin
Enter point
Record AREA
PIN
17 2432.473 21392904
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Statistics
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STATISTICS generates summary statistics from items in an INFO data file and may
save them to a new INFO data file. Usage here is for ArcPlot.
– Use STATISTICS to determine SUM, MEAN, MAXIMUM, MINIMUM, or
STANDARD DEVIATION for an item or items in a Tables data file.
– STATISTICS <cover> <feature_class> {case_item} {out_info_file}
• Case-Item is NOT the variable for which you want statistics. It is a grouping (BY)
variable.
• Separate statistics are produced for each set of records with same value on this variable.
• Variable(s) for which you want stats are named within the dialog (see next slide)
• Be sure you have the desired “selection set” of records before requesting statistics
– Arcplot: statistics europe polys
• Since no {case_item} is specified, one set of statistics is calculated using all currently
selected records.
– Arcplot: statistics streets arcs street-name streets.sta
• For each unique value of street-name {case_item}, summary statistics are calculated and
placed in the streets.sta {output_file}.
– Arcplot: statistics aircover points # aircover.sta
• Since no {case_item} is specified, one set of statistics is calculated using all currently
selected records and saved in the file aircover.sta
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Statistics example
In this example, you have a polygon coverage europe containing a variable (item)
area for which you want stats. No {case-item} or {output file} is used.
Arcplot: statistics europe polys
Enter statistical expressions. Type END or blank line to end.
Statistics: sum area
Statistics: mean area
Statistics: max area
Statistics: min area
Statistics: end
1
FREQUENCY = 87
SUM-AREA = 1721234.892
MEAN-AREA = 21671.100
MAX-AREA = 202398.519
MIN_AREA =
675.325
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